I'm not sure it's all that useful a term, but the idea seems usually to be that an idiom of "folk character" derives from the common practice of some specific regional and ethnic group, accessible in some way to everybody within it. That cuts out cultivated idioms that transcend geographic and ethnic boundaries or specifically cater for class minorities - Gregorian chant, Ottoman classical music, gypsy fiddle showpieces, pibroch or techno, for a few.

Since music interacts with political economy, ethnography and geography in so many different ways in different times and places, I can't see how such a general categorization could be much use to anybody.